The present exemplary embodiment relates to a boss which resists stripping of internal threads with repeated threading of a screw. It finds particular application in conjunction with the attachment of components to a printer chassis, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Office machines, such as printers, copiers, and fax machines generally include a chassis formed of metal, on which a variety of components, such as fans, motors, drive systems, and the like, are separately mounted. The chassis is provided with bosses, which project from a generally planar frame for receiving screws by which the various components are attached. Injection molded plastic provides a cost effective alternative for forming components which have traditionally been formed of metal.
The screws used to attach the components to the chassis may be self-tapping or form threading screws which create a thread in the boss during the first insertion. When the component is replaced or repaired, the screws are removed and replaced along with the new or repaired component. Sometimes, the replaced screw does not engage the original thread in the boss and a second thread is formed. This reduces the amount of material available in the boss for creating new threads. The screw reaches higher torques as it is beginning to clamp. The reduced amount of material may be unable to handle the loads and it is stripped off, creating a stripped screw boss without threads.
Where multiple bosses are used and the screws are inserted and removed multiple times, the chances of a single boss failure preventing reassembly of the component are multiplied. For example, there may be twenty bosses on a chassis. For bosses having a failure rate of only 0.2%, and if five of the screws are replaced with two insertions each, the chance of one of the components being inadequately mounted is 1%. Since the screws may be rethreaded one or more times even before the printer leaves the factory, the possibility of a component mounting failure during a first or second service operation can be significant. While hand threading of screws can reduce the incidence of boss thread stripping, it is generally not a cost effective option in most manufacturing and service environments.
Various methods have been developed for reusing a boss which has had its thread stripped. In one method, the original screw is replaced with a screw with a larger thread diameter, which creates a new thread in the boss. In another method, part of the boss is cut out and replaced with an insert part. The insert part may be molded in or sonically welded to the chassis. In the field, however, the technician may not always have extra-enlarged screws available. Creating an insert is a time consuming process and is not always effective in terms of boss reliability. Thus, there are instances where an entire printer is replaced due to failure of a single boss.
The present exemplary embodiment contemplates a new and improved boss and method of use, which overcomes the above-referenced problems and others.